Problem with QGIS - Attribute names are shown as _1,_2,_3 ... and Strings are displayed as "NULL"

I have the followings problem: Whenever I look at the attribute table or at the attribute dialog for a shape, the attribute names are shown as _1,_2,_3, ... In the attributes, Strings are generally displayed as "NULL". Numbers are fine. This happens to all shapefiles and it has not been a Problem in the beginning but at some point it startet. I unintalled Qgis several times and reinstalled different versions (2.2,2.6,2.8) from different sources (Ubuntu repositories, QGIS repositories) ... the problem remained. I also uninstalled grass libraries ... the problem remained. I reinstalled QT ... the problem remained. Any suggestion? With that problem, QGIS is hardly usable :(

I'm using Ubuntu 64-bit. The problem startet under version 14.04, but survived an update to 14.10.

Thanks!

p.s.:

In the message window, I found one warning by python:

warning:/usr/lib/python2.7/dist-packages/PyQt4/uic/uiparser.py:890: PendingDeprecationWarning: This method will be removed in future versions. Use 'elem.iter()' or 'list(elem.iter())' instead.

for include in elem.getiterator("include"):

traceback: File "", line 1, in

File "/usr/lib/python2.7/dist-packages/qgis/utils.py", line 219, in startPlugin

plugins[packageName] = package.classFactory(iface)

File "/usr/share/qgis/python/plugins/MetaSearch/init.py", line 28, in classFactory

from MetaSearch.plugin import MetaSearchPlugin

File "/usr/lib/python2.7/dist-packages/qgis/utils.py", line 478, in _import

mod = _builtin_import(name, globals, locals, fromlist, level)

File "/usr/share/qgis/python/plugins/MetaSearch/plugin.py", line 31, in

from MetaSearch.dialogs.maindialog import MetaSearchDialog

File "/usr/lib/python2.7/dist-packages/qgis/utils.py", line 478, in _import

mod = _builtin_import(name, globals, locals, fromlist, level)

File "/usr/share/qgis/python/plugins/MetaSearch/dialogs/maindialog.py", line 49, in

from MetaSearch.dialogs.manageconnectionsdialog import ManageConnectionsDialog

File "/usr/lib/python2.7/dist-packages/qgis/utils.py", line 478, in _import

mod = _builtin_import(name, globals, locals, fromlist, level)

File "/usr/share/qgis/python/plugins/MetaSearch/dialogs/manageconnectionsdialog.py", line 39, in

BASE_CLASS = get_ui_class('manageconnectionsdialog.ui')

File "/usr/share/qgis/python/plugins/MetaSearch/util.py", line 59, in get_ui_class

return loadUiType(ui_file_full)[0]

File "/usr/lib/python2.7/dist-packages/PyQt4/uic/init.py", line 210, in loadUiType

winfo = compiler.UICompiler().compileUi(uifile, code_string, from_imports, resource_suffix)

File "/usr/lib/python2.7/dist-packages/PyQt4/uic/Compiler/compiler.py", line 139, in compileUi

w = self.parse(input_stream, resource_suffix)

File "/usr/lib/python2.7/dist-packages/PyQt4/uic/uiparser.py", line 992, in parse

actor(elem)

File "/usr/lib/python2.7/dist-packages/PyQt4/uic/uiparser.py", line 890, in readResources

for include in elem.getiterator("include"):

I don't think that this causes the problem, but I don't know.